Infrared spectroscopy is a generally known basis for analysis. In some forms, infrared spectroscopy measures the electromagnetic radiation (typical wavelength range of 0.7-25 μm) that a substance absorbs at various wavelengths, though other methods measure other effects a substance has on incident light. Absorption phenomena can be related to molecular vibrations and shifts in energy levels of individual atoms or electrons within a molecule. These phenomena cause the absorbing molecule or atom to switch to a higher energy state. Absorption occurs most frequently in limited ranges of wavelengths that are based upon the molecular structure of the species present in the measured sample. Thus, for light passing through a substance at several wavelengths, the substance will absorb a greater percentage of photons at certain wavelengths than it will at others. The characterization of substances by their spectral absorption characteristics is well known.
At the molecular level, many primary vibrational transitions occur in the mid-infrared wavelength region (i.e., wavelengths between 2.5-6 μm). However, for some measurements, such as the non-invasive measurement of analytes in tissue, use of the mid-infrared region can be problematic because molecules with strong absorbance properties (e.g. water) can result in the total absorption of virtually all light introduced to the sample being measured. The problem can be overcome through the use of shorter wavelengths (typically in the near infrared region of 0.7-2.5 μm) where weaker overtones and combinations of the mid-infrared vibrations exist. Thus, the near-infrared region is often employed in such situations as it preserves the qualitative and quantitative properties of mid-infrared measurements while helping to alleviate the problem of total light absorption.
As mentioned above, alcohol and other analytes absorb light at multiple frequencies in both the mid- and near-infrared range. Due to the overlapping nature of these absorption bands, reliable analyte measurements can be very difficult if only a single frequency or wavelength is used for analysis. Thus, analysis of spectral data often incorporates absorption characteristics at several wavelengths, which enables sensitive and selective analyte measurements. In multi-wavelength spectroscopy, multivariate analysis techniques are often used to empirically determine the relationship between measured spectra and a property of interest (e.g. analyte concentration).
Advances in optical materials and multivariate algorithms over the last several decades have created the potential for expanding spectroscopic measurements into new areas of interest. One such area is the noninvasive measurement of analytes such as alcohol in humans. The application of spectroscopic techniques to measurement of analyte properties such as alcohol concentration can be complicated by the presence of interferents. Interferents can mask or obscure the response due to alcohol, leading to false or misleading results. This problem is made more significant by the motivation of individuals to confound alcohol measurements in punitive settings such as traffic stops. A similar problem exists in spectroscopic measurement of other analytes, where, for example, interferents such as lotion or insect repellant can contribute to inaccurate results such as glucose concentration measurements. There is a need for apparatuses and methods to mitigate the effects of foreign interferents on spectroscopic measurements.